FIG. 1 shows an example of the representative structure of a motor drive device of a motor to drive a feed shaft and a main shaft of a machine tool, arms of an industrial machine or an industrial robot, or the like. FIG. 1 is a circuit for the motor drive device that produces three-phase alternating current power from a direct current power supply. By applying drive signals to gates of power elements Tr1 to Tr6 that are switching elements, which consist of three pairs of upper and lower power elements each for a U-phase, a V-phase, and a W-phase contained in an inverter 100, the power elements are turned on and off to feed electric power to a motor 10. Diodes D1 to D6 are connected in parallel to the power elements Tr1 to Tr6, respectively. A current flowing through the motor 10 is detected by a current detection unit 200, which includes a U-phase current detector 201 and a V-phase current detector 202, and fed back to a current controller 300.
At this time, the pair of upper and lower power elements are alternately turned on and off, and a period of time (dead time) in which both of the power elements are necessarily turned off is provided at the instant of switching the turn on and off of the two power elements of a certain phase (see FIG. 2). This is because turning on the pair of upper and lower power elements at the same time causes the upper and lower power elements to short out, and a large current flowing through the power elements breaks the power elements.
The dead time will be described with reference to FIG. 2. FIG. 2 shows waveforms of signals A and B. The signal A (A′) is applied to the gate of the U-phase upper arm power element Tr1, and the signal B (B′) is applied to the gate of the U-phase lower arm power element Tr2. The power element is turned on when the signal A or B is at a high level, and turned off when the signal A or B is at a low level. As described above, when both of the signals A and B are at the high level, the U-phase upper arm power element Tr1 and the U-phase lower arm power element Tr2 are turned on at the same time, and the power elements in a short state may possibly break down. Accordingly, in order to prevent the signals A and B from being at the high level simultaneously, the dead time that is the period of time in which both of the upper arm power element Tr1 and the lower arm power element Tr2 are turned off, in other words, both of the signals A and B are at the low level provided at the instant of switching the turn on and off of the upper arm power element Tr1 and the lower arm power element Tr2, which compose an output stage of the inverter 100. This dead time has a width for a certain duration, for example, a dead time Tset set by a gate drive command generator 400.
The drive motor applies a voltage to the motor and controls a current flowing through the motor in order to rotate the motor at a desired rotating speed or stop the motor in a desired position. However, the presence of the dead time, as described above, interferes with application of a desired voltage to the motor and a flow of a desired current through the motor.
A technique (hereinafter referred to as “dead time correction”) for correcting the effect of the dead time is known in which a voltage is added to a voltage command by a deficiency owing to the dead time or a voltage is subtracted from the voltage command by an excess owing thereto (For example, Japanese Patent Application Laid-Open No. 2012-254682 (JP 2012-254682 A)).
The proper functioning of the dead time correction is predicated upon a proper grasp of the dead time. In an actual motor drive device, as shown in FIG. 1, the signal A and the signal B, which are produced by the gate drive command generator 400 to turn on and off the power elements, pass through an upper arm gate drive circuit 501 and a lower arm gate drive circuit 502 included in a gate drive circuit 500, respectively, for isolation and amplification, and enter the gates of the power elements as a signal A′ and a signal B′ in actual fact. As a result, the signal A′ and the signal B′, being signals to turn on and off the power elements, are delayed in passing through the gate drive circuit 500. This delay is sensitive to variations in properties of circuit components and the like, and is inconstant.
A variation in the dead time owing to passing through the gate drive circuit 500, as described above, will be described with reference to the drawings. FIG. 3 shows the waveforms of the signal A and the signal B, which are produced by the gate drive command generator 400 to turn on and off the power elements, and the signal A′ and the signal B′, which are isolated and amplified through gate drive circuit 500 and actually inputted to the gates of the power elements Tr1 and Tr2. The signal A′ is delayed by a time Ta relative to the signal A by passing through the upper arm gate drive circuit 501. In a like manner, the signal B′ is delayed by a time Tb relative to the signal B by passing through the lower arm gate drive circuit 502. When Tset refers to the dead time of the signal A and the signal B set by the gate drive command generator 400, a dead time T of the signal A′ and the signal B′ at the gates of the upper arm and lower arm power elements, respectively, is represented by T=Tset+(Ta−Tb). It should be noted that, as shown in FIG. 3, Ta is not necessarily equal to Tb. Accordingly, there occurs a case where T is unequal to Tset.
In this manner, as shown in FIG. 3, there is a case in which the dead time T at the gates of the power elements is different from the value Tset set by the gate drive command generator 400. Furthermore, time for the power elements to turn on and off in response to the signals inputted to the gates vary from one power element to another, and is represented as difference in the dead time in output stages of the power elements.
Namely, even if the gate drive command generator 400 applies the dead time correction to the dead time Tset set by the gate drive command generator 400, the dead time correction is not necessarily appropriate in the output stages of the power elements. If anything, such a correction may bring about an excess or a deficiency of the correction, and adversely affect current control of the motor. According to the conventional technique, as described above, it is difficult to correctly grasp the dead time T in the output stages of the power elements, and therefore the dead time correction cannot be performed in an appropriate manner.
For this problem, it is conceivable to add a voltage measurement circuit or the like in the output stage of the power element for the purpose of measuring an actual dead time in the output stages of the power elements. However, the addition of the circuit causes a disadvantage in mount size and cost. There are also various problems e.g. a requirement for isolation between a primary circuit and a secondary circuit, a measurement error in the circuit, and the like.
An object of the present invention is to provide a motor drive device that can correctly estimate the dead time in the output stages of the power elements with the use of an existing circuit without any additional circuit.